Higher storage bit densities in magnetic media used in disk drives have reduced the size (volume) of data cells to the point where the cell dimensions are limited by the grain size of the magnetic material. Although grain size can be reduced further, the data stored within the cells may not be thermally stable. That is, random thermal fluctuations at ambient temperatures may be sufficient to erase data. This state is described as the superparamagnetic limit, which determines the maximum theoretical storage density for a given magnetic media. This limit may be raised by increasing the coercivity of the magnetic media or by lowering the temperature. Lowering the temperature may not always be practical when designing hard disk drives for commercial and consumer use. Raising the coercivity, on the other hand, may result in a requirement for write heads that incorporate higher magnetic moment materials, or techniques such as perpendicular recording (or both).
One additional solution has been proposed, which uses heat to lower the effective coercivity of a localized region on the magnetic media surface and writes data within this heated region. The data state becomes “fixed” upon cooling the media to ambient temperatures. This technique is broadly referred to as “thermally assisted (magnetic) recording” (TAR or TAMR), “energy assisted magnetic recording” (EAMR), or “heat-assisted magnetic recording” (HAMR). The term “HAMR” is used herein to refer to all of TAR, TAMR, EAMR, and HAMR.
In HAMR, a magnetic recording material with high magneto-crystalline anisotropy (Ku) is heated locally during writing to lower the coercivity enough for writing to occur, but the coercivity/anisotropy is high enough that the recorded bits are thermally stable at the ambient temperature of the disk drive (i.e., the normal operating or “room” temperature of approximately 15-30 degrees Celsius). In some proposed HAMR systems, the magnetic recording material is heated to near or above its Curie temperature. The recorded data may then be read back at ambient temperature by a conventional magnetoresistive read head. HAMR disk drives have been proposed for both conventional continuous media, wherein the magnetic recording material is a continuous layer on the disk, and for bit-patterned media (BPM), in which the magnetic recording material is patterned into discrete data islands or “bits.”
One type of HAMR disk drive uses a laser source and an optical waveguide coupled to a near-field transducer (NFT) for heating the recording material on the disk. A “near-field” transducer refers to “near-field optics,” wherein light is passed through a first element with subwavelength features and the light is coupled to a second element, such as a substrate (e.g., of a magnetic recording medium), located a subwavelength distance from the first element. The NFT is typically located at the air-bearing surface (ABS) of an air-bearing slider that also supports the read/write head and rides or “flies” above the disk surface. A NFT may have a generally triangular output end, such that an evanescent wave generated at a surface of the waveguide couples to surface plasmons excited on the surface of the NFT, and a strong optical near-field is generated at the apex of the triangular output end.
Because of its function, the NFT can reach high temperatures, which, if left uncorrected or continuing for too long a period of time, can cause HAMR drive failures.
Many data storage devices, such as hard disk drives, operate in a standard air (e.g., nitrogen, oxygen, and water vapor mixture) atmosphere. Spinning disks in hard disk drives at high revolutions per minute against the friction of an air atmosphere is largely inefficient and requires a certain amount of power. As an alternative, data storage devices, such as hard disk drives, can be filled with a lower-density gas, such as helium, and sealed to control and maintain the device's internal environment. Sealing mitigates or prevents leakage of internal gases from within the storage device. The use of helium, which has a density that is approximately one-seventh that of air, reduces friction and vibration in the device, thereby creating less drag and turbulence. Consequently, by running the hard disk drive in a less-dense atmosphere, such as an atmosphere of helium or a helium mixture, friction on the disk is reduced, thereby causing the disk to require less power in order to spin at a similar rate as the disks in drives that operate in standard air conditions. The use of helium also reduces the operating temperature of the drive, as well as noise generated by the drive.
The lower ambient pressure in sealed helium drives poses challenges to ABS designs, however. For example, the intensive heating used in HAMR can cause desorption of organic gas phase contaminants and lubricant molecules on the media surface, and then accumulation of those materials at the NFT area. The material buildup, known as smear, has been found to form around the NFT. Smear accumulation can affect the reliability of the NFT and also can cause head-disk interface issues, such as touch down power change in the drive.
Therefore, there is a need in the art for solutions that mitigate the effects of smear from the write head and, in particular, from the NFT area of a HAMR write head.